A couple are caught out in the rain in Porthcawl, south Wales. Photograph: Matt Cardy/Getty Images

Revellers going to the Isle of Wight festival and the Hackney Weekend would be advised to wear their wellies and pack a cagoule, with wet weather expected across the UK as the unusually sodden June weather looks set to continue.

Flood warnings have been issued in parts of the north-west as the region is expected to be battered by heavy rain. Up to 100mm (4in) could fall in some places, with 50-60mm expected across much of Greater Manchester, Merseyside and Lancashire.

Hundreds of revellers have been forced to sleep in their cars after traffic became gridlocked when the Isle of Wight festival site turned into a mudbath caused by heavy rain.

The queues caused ferry companies to suspend their services and about 600 people were stranded last night on ferries on the Solent as passengers could not disembark because of traffic build-up on the island.

The problems started after heavy rain fell on Thursday, waterlogging the event's car parks at Seaclose park, Newport, which became churned up with mud as cars began to arrive.

A festival spokeswoman said: "If you are on the mainland we are advising people not to travel by car.

"There is a ferry service for foot passengers and this will be the easiest route. We are sorry for any disruptions caused."

More than 50,000 people are travelling to the festival, which is being headlined by Pearl Jam, Tom Petty and the Heartbreakers and Bruce Springsteen and the E Street Band.

Festivalgoers are unlikely to be able to fully enjoy an artificial giant beach by the river Medina next to the site at Seaclose park. Music fans going to the BBC's free concert in Hackney, east London, are also at risk of getting wet.

The Environment Agency warned heavy rain could lead to surface water flooding of roads and land, and there is also a risk of floods from rivers in built-up areas that are quickly affected by large amounts of rainfall.

People in Greater Manchester, Liverpool, the Wirral, North Cheshire and Lancashire should be prepared for the possibility of flooding from Friday afternoon through to Saturday, said the agency. A team of agency experts will be on hand in the north-west over the weekend to check flood defences, clear river blockages and monitor river levels to try to reduce the risk of flooding.

Further down the country, parts of south-east England, East Anglia, the north and east Midlands and north and west Wales are also on alert for localised surface water flooding on Friday and into the weekend.

The Meteogroup forecaster Matt Dobson said the north-west and Cheshire had already been lashed with rain in the past 24 hours. "The worst weather overnight has been across the north-west and into Cheshire, where between 10-20mm fell. Central and southern Scotland were also badly hit," he said. "Throughout the day the rain really is going to develop from south-west Scotland, across Northern Ireland down into north-west England. It will be very wet in those areas, with the worst of it in Cumbria and the western Pennines, from Manchester up to Cumbria and across into south-west Scotland, and in north-west Wales as well."

He added: "There will be 20-30mm of rain quite widely, with maybe as much as 50-60mm of rain over upland areas such as the fells of Cumbria and the western Pennines."

Further serious weather warnings and flood alerts are expected through the weekend and those in affected areas are urged to check the Environment Agency website and Twitter feed for the latest updates and warnings. People are being warned to stay away from swollen rivers and not to drive through floodwater.

Parts of England and Wales have been hit by flooding in recent weeks after unusually wet weather followed two dry winters in a row that had left swaths of England in drought.